1. Field of the invention
The present invention relates to a radial tire with steel belt plies, and more particularly to a radial tire in which at least a portion of a tread rubber adjacent to the outside of the steel belt plies is composed of a center rubber for middle part of the tread and a side rubber disposed on both sides in the width direction of the tire, are formed of, i.e., two kinds of rubber compositions.
2. Description of the prior art
The tread of a radial tire is hard to deform since it is usually reinforced by providing the outside of a carcas ply comprising cords disposed radially with steel belt plies of high rigidity. Accordingly, when the tire is used on an unpaved road, the effect of dispersing stress by wrapping a trodden stone with a tread, i.e., a so-called envelope effect is relatively small, and the tire is easily cut. Particularly in the middle part of the tread, such envelope effect is small and at this part of the tread, the problem of cutting frequently occurs. When a large number of small cuts occur, the tread rubber is chipped and abrasion is accelerated. When such cuts reach a belt, moisture reaches the belt and causes the belt to rust, and finally causes a separation between the belt and the rubber. In the case of forming the tread portion into a two-layer construction comprising a cap and base and using a rubber of low heat build-up for a base rubber, in particular for the purpose of preventing the belt separation, since there is a reverse correlation between the low heat build-up of rubber and the cut resistance thereof as shown in FIG. 3, the cut resistance is not sufficiently obtained. Thus it is often the case that the belt separation occurs due to an invasion of moisture from the cut.
In view of the foregoing an attempt was proposed in Japanese laid-open Patent Publication (unexamined) No. 60-15203. According to this proposal, in a radial tire of two-layer construction comprising a cap and a base, a rubber of superior shock cut resistance is used for the middle portion of the crown of the base rubber, while a rubber of low heat build-up is used for a shoulder portion thereof, and wherein three different kinds of rubbers, each reinforced with carbon alone, are applied to cap rubber, base rubber middle portion and base rubber shoulder portion, respectively, to form a tread. However, even in the case of using different kinds of rubbers for the middle portion and the shoulder portion of the base rubber in the tread rubber of two-layer construction as proposed by this prior art, insofar as the tread is composed of the three different rubbers reinforced with carbon alone, it is difficult to improve the cut resistance while restraining the belt separation due to heat bus. Moreover, in the case of forming the tread rubber of plural kinds of rubbers, it is necessary to form it by extrusion molding. But it is quite difficult to arrange three kinds of rubbers at four places and to extrude them simultaneously for molding, from the view point of a manufacturing process.